1. Field of the Invention
The present invention relates to improvement of a focus detecting device for use in film cameras, digital color cameras and other cameras, and further to improvement of an image pickup (imaging) unit internally including the same focus detecting device.
2. Description of the Related Art
In digital color cameras, in response to depression of a release button, a solid-state image pickup element such as a CCD or a CMOS sensor is exposed to a field image for a desired period of time, and an image signal representative of a still image in one image plane is converted into a digital signal and is subjected to predetermined image signal processing such as YC processing to obtain an image signal of a predetermined format.
The digital image signal indicative of the image picked up is recorded in a semiconductor memory according to frame image. The image signal recorded is, as occasion calls, read out to be displayed, or reproduced into a printable signal and outputted to a monitor or the like to be displayed.
In a case in which the image signal picked up is displayed as an image in the real time, the image signal can be displayed instead of being stored in the semiconductor memory, and it can be displayed as an image varying dynamically at all times. On the other hand, for the display of a still image, the image signal is temporarily stored in a semiconductor memory such as VRAM and is read out from that semiconductor memory to be displayed as a still image in a monitor such as a liquid crystal display. Accordingly, in a case in which a plurality of still images are taken and reproduced/displayed or printed afterwards, the image signal is temporarily stored as a VRAM image signal in an erasable-type program ROM such as a flash memory or an EEPROM, a memory stick, a memory card or the like, with the VRAM image signal being stored semipermanently in a semiconductor memory, a memory tape or the like.
However, in the case of a digital color camera offering a high-quality image, a need for more important factors exists at the image pickup. That is, as the factors, there are a focus detecting means at the image pickup and the lens alignment for placing the focus on a subject.
So far, a digital color camera has employed a contrast detection type focus detecting device. The contrast detection type obtains the sharpness of an object image, formed through an image pickup optical system, by evaluating an output of a solid-state image pickup element on the basis of a predetermined function for adjusting the position of the image pickup optical system on the optical axis so that the function value assumes a maximum value. Among the evaluation functions, for example, there are a function that adds the absolute value of the difference between the adjacent luminance signals in a focus detecting area, a function that adds the square of the difference between the adjacent luminance signals in a focus detecting area and a function that similarly processes the difference between the adjacent signals at the level of each of R, G and B image signals.
In general, in such a contrast detection type focus detecting device, since the evaluation function is obtained while the position of the image pickup optical system on the optical axis is shifted bit by bit, the time for the evaluation function processing becomes necessary, and the focusing operation takes time until the in-focus condition is reached.
In addition, as disclosed in U.S. Pat. No. 4,410,804, there has been known an image pickup unit incorporating the so-called phase difference detection type focus detecting device in which one pair of or two pairs of light-receiving sections are provided according to two-dimensionally arranged microlens array to divide a pupil of an image pickup optical system by projecting the light-receiving sections on the pupil through the microlenses. The phase difference type forms object images through the use of two light beams passing through different portions of a pupil of an image pickup optical system for detecting the positional phase difference between the two object images on the basis of outputs of image pickup elements to convert the detected positional phase difference into a defocus quantity of the image pickup optical system.
Since the phase difference detection type focus detecting device is capable of detecting the defocus quantity, it can considerably shorten the time taken until the in-focus condition is reached, as compared with the contrast detection type.
A solid-state image pickup element with the structure disclosed in U.S. Pat. No. 4,410,804 employs a microlens array to form one pair of or two pairs of images through the use of a light beam passing through a portion of a pupil of the image pickup optical system. The power of each microlens is set so the each of the light-receiving sections of the image pickup element is projected to an exit pupil of the image pickup optical system 24, while each light-receiving section of the image pickup element and the exit pupil are in conjugate relation to each other.
In this case, for establishing linear relationship between the light quantity incident on the light-receiving sections and the opening degree (aperture) of the diaphragm (iris) of the image pickup optical system, the projection magnification may be selected so that a projected image on each light-receiving section is larger than the exit pupil at the release of the diaphragm of the image pickup optical system. Thus, knowing the subject luminance and the sensitivity of the image pickup element, the lens (diaphragm) opening degree and the shutter speed are calculable in the same manner as that for a film camera. That is, the incident light quantity shows proportional relation to the opening area, which satisfies the calculation according to the APEX mode (Additive System of Photographic Exposure).
However, the defocused condition creates a problem in that, since the configuration of the pupil divided is superimposed on a blurred image, difficulty is encountered in detecting a large-defocus condition.
In addition, in this mode, as another factor to increase the detection error on the defocus quantity, there is an error resulting from manufacturing of the microlens. As mentioned above, the microlens projects a light-receiving section to the exit pupil of the image pickup optical system. Assuming that the projection position is different according to pixel, the phase shift quantity at the defocus varies according to pixel. This provides a larger effect as the defocus quantity increases. However, the microlens has a very fine structure; therefore, in fact, the difference among the microlenses stemming from the manufacturing may be compromised to some degree.